• Field Notes 22.08.2013

    Fasteners are one of the great variables with which glazing pros have to be up to speed.  Obviously germane to fastener selection is the application and location (shop assembly or field installation) where the screw is to be used. How big a load will it carry? Is that load in shear or tension? Is the fastener to be installed in a wet or dry area?

    A chapter could be written about each of the screw selection factors listed below, and there are better resources than this blog for picking up the information. Wander the local hardware store screw section and you’ll see the variations that go into selecting the correct fastener.  As one who makes three trips to the hardware store on most Saturdays (first to get what I think I need, second to get what I really need, and third to get it in the correct size / length / or quantity), I can tell you there’s a plethora of fasteners to choose from.

    All of the factors below have to be figured into screw selection:

    1. How is it being driven? That affects the type of head required.
    2. Will there be access to drive the screw? If it’s a hex head, is there allowance made in the placement of the screw for a socket or ratchet? If internally driven (think Phillips, Allen, or Torx-drive), can the driver still have room with a screw gun to tighten the fastener?
    3. What’s the diameter of the screw? That depends on the work that’s being done and / or the spacing.
    4. Generally, the more frequent the screw, the smaller the diameter. But, that also involves more labor and / or more effort (shop or field) to do any preparation of the material to receive the screw, another variable in the screw selection.
    5. What length is required?  How much material is it grabbing, which is called its “working length?” In bi-metal screws, this is critical as the length has to be long enough that the material being connected is in the “working length.” Other fasteners have to grab the material past any tapered point.
    6. Will the holes be pre-drilled in the material?  That affects the tip of the screw or point selection: self-drilling screws vs. Type A/B/AB thread. Or thread-forming screws?
    7. What type of thread?  Machine screw, sheet metal screw, thread forming, thread cutting, all have different threads.
    8. What’s the material?  Stainless or carbon steel?  High strength (usually for bolts or anchoring conditions)?
    9. Is it coated?  Zinc, galvanized, or special coating unique to the screw or manufacturer?  Or uncoated, as in the case with stainless steel?  For the older generation, it looks like cadmium plated QQP-416 screws have gone the way of the dodo due to environmental concerns.  But, they were to this writer like mother’s milk back in the day…
    10. How is to be locked in place, if at all?  Lock washer or Loctite?  Gnarled washer on the underside of the screw head?  Locking nuts, plastic plugs, etc.
    11. How is it to be sealed, if it needs to be sealed? Application of sealant over the head or does the thread point need to be sealed? (which is a tricky proposition, not wholly recommended, but sometimes the only available option). Or is a rubber or sealant washer placed by the manufacturer under the head of the fastener, thus becoming self sealing? Is whatever method chosen sufficient to stop leaks?

    With all the variables (and any that are missing) picking the right screw often feels like choosing the right lottery numbers. For the jackpot a couple of weeks ago, there was a 1/175,000,000 chance of winning. The chances the right fastener is selected are a lot better, but get the wrong one and the field crews will hate you, fabrication and / or field labor can be affected, all of which obviously impacts the bottom line.

    Keeping the variety of fasteners to a minimum is a challenge, but also lessens the chance the field crews will inadvertently put the wrong screw in the assembly. Minimizing the selection is easy to do if the designer can make the most out of any one screw. Granted, anchor fasteners are going to be different than fasteners locking a sheet metal panel in an opening. But, that’s a large difference, not so much if there are five different lengths of the same type of screw, or three different diameters of the same hex washer head sheet metal screw, for example.

    And, there’s the added responsibility that the fastener has to be absolutely correctly identified on the shop drawings and the structural calculations. Equally important, the shop and field crews have to correctly use the right screw at a given location. The only way they know that is by following the shop drawings. None of us want walls falling apart. Use the wrong screw in the wrong location, and it’s tough to sleep at night.

    Screw descriptions can be pretty long if each and every fastener is called out on each and every detail.  A good case can be made for making a fastener index at the front of the shop drawing set, where each screw is shown in detail, described, and given a reference number. That reference number in a bubble (or other suitable symbol) is shown on the detail, saving space at the detail for other notes.  Other materials can be indexed this way (gaskets, for instance), so given the variety of fasteners, and having to type out their descriptions every time, fasteners come under the “no-brainer” category for indexing.

    So, I’d like to make a case for a standard for how fasteners are called out on shops.  After reviewing  and preparing more shops than I care to think about, I have seen this done so inconsistently, maybe it’s something GANA or AAMA can write a standard for, or it could be added to GANA’s standards for what constitutes good shop drawings.

    At least for conversation purposes, can the following work for a screw standard description?:

    Diameter x length x material x head type / driver x thread type x tip x finish x spacing (if necessary) x any special feature that might be required.

    The arrangement might vary depending on the screw, but consistency is the object here, and that’s not met when some or most of the description is missing, which can result in the wrong screw being used.

    And hopefully, if you win the next lottery, I really don’t want much. Just a new car, under $30,000? By saving job costs from the field crews not losing screws (see last week’s blog), you’ll have the money saved, right?

    Posted by Blogger @ 11:17 am

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